This invention relates to hot wire type arc welding systems, and more particularly to a protecting circuit for switching elements of a DC TIG welding source, which are provided for heating a filler wire at a constant voltage.
A system of this type is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a DC voltage source; 2 and 3, first and second switching elements connected to the positive terminal of the voltage source 1, which are preferably parallel-connected transistors; 4, a first DC reactor connected to the output of the first switching element; 5, a second DC reactor connected to the output of the second switching element 3, 6, a first power source output terminal connected to the output of the first DC reactor; 7, a second power source output terminal connected to the output of the second DC reactor 5; 8, a current detector connected to the negative terminal of the DC voltage source 1; 9, a third power source output terminal connected to the output of the current detector 8; and 10, a control circuit for receiving signals from the first power source output terminal 6, the second power source output terminal 7 and the current detector 8, to apply drive signals to the first and second switching elements 2 and 3. Further in FIG. 1, reference numeral 11 designates a current supplying tip connected to the first power source output terminal 6; and 12 is a filler wire fed through the current supplying tip by a feeding unit (not shown). A workpiece 13 is coupled to a second power source output terminal 7; and a welding torch 14 is connected to the third power source output terminal. A torch electrode 15 is connected to the end of the welding torch 14.
The operation of the system thus organized will now be described.
When welding starts, i.e., when arcs are generated between the torch electrode 15 of the welding torch 14 and the workpiece 13 and the filler wire 12 is fed towards the workpiece 13 to cause the short-circuiting therebetween, current flows from the DC voltage source 1 through the first switching element 2, the current supplying tip 11, the filler wire 12, the workpiece 13 and the torch electrode 15 of the welding torch 14, that is, heating current I.sub.1 flows in the filler wire 12. In this operation, an instruction signal is applied to the first switching element 2 so that the voltage at the heated part of the filler wire 12, which is fed back to the control circuit 10 through the first power source output terminal 6, is maintained constant. As a result, the first switching element 2 is repeatedly turned on and off. The heating voltage to the filler wire 12 is smoothed by the first DC reactor 4 into a substantially pure DC voltage, thus being maintained constant. On the other hand, welding current I.sub.2 flows through the second switching element 3, the second DC reactor 5, the second power source output terminal 7, the workpiece 13, the torch electrode 15 and the first power source output terminal 9. Accordingly, the actual welding current is the sum of the heating current I.sub.1 of the filler wire 12 and the above-described welding current I.sub.2 ; that is, welding current (I.sub.1 +I.sub.2) flows through the current detector 8. The welding current (I.sub.1 +I.sub.2), being detected by the current detector 8, is applied to the control circuit 10. The control circuit 10 applies an instruction signal to the second switching element 3 so that the welding current (I.sub.1 +I.sub.2) is maintained constant. As a result, the second switching element 3 is repeatedly turned on and off. In this operation, the actual welding current (I.sub.1 +I.sub.2) flowing in the welding arc is smoothed by the second DC reactor 5 into a substantially pure direct current, thus being maintained constant. While the workpiece 13 is made molten by the heat of the arcs generated between the torch electrode 15 and the workpiece 13, the filler wire 12 is fed onto the workpiece while being heated, so that the filler wire 12 is readily melted into the workpiece, i.e., welding is readily achieved. Even, in this connection, if the hand is shaken or the configuration of the workpiece 13 is changed more or less to vary the length of the heated part of the filler wire 12, the filler wire 12 is never heated excessively or insufficiently, i.e., welding is carried out stably, because the heating voltage is maintained constant as described above.
The conventional DC TIG welding power source is designed as described above. Accordingly, when a short occurs between the current supplying tip 11 and the torch electrode 15, or the filler wire, being short in dimension, is short-circuited with the torch electrode 15, a considerably large short-circuit current flows because the heating voltage of the filler wire is stabilized as described above. The short-circuit current may damage the first switching element 2, by exceeding the rating of the latter.